Electronic systems especially those for communications applications operated at radio frequencies (RF) require small bandpass filters and oscillators. The oscillators are for generation of RF signals and the bandpass filters are for selection (transmitting or receiving) of signals within certain bandwidth (BW) at a given frequency. Some examples of the systems include global positioning systems (GPS); mobile telecommunication systems consist of: Global Systems for Mobile Communications (GSM), personal communication service (PCS), Universal Mobile Telecommunications System (UMTS), Long Term Evolution Technology (LTE); data transfer units containing: Bluetooth, Wireless Local Area Network (WLAN); satellite broadcasting and future traffic control communications. They also include other high frequency systems for air and space vehicles.
RF bandpass filters are fabricated using different technologies: (a) ceramic filters based on dielectric resonators; (b) filters based on surface acoustic wave resonators (SAW); and (c) filters using thin film bulk acoustic wave resonators (FBAR). Both SAW and FBAR are used when dimensions of the systems are limited. Presently, SAW devices are mainly used in volume applications at frequencies below 2 GHz whereas FBARs are dominant in systems operated at frequencies of 2 to 4 GHz or higher. Due to large volumes, current SAW or FBAR RF filters in handsets are manufactured by microelectronic fabrication processes on wafers using piezoelectric materials such as LiNbO3 (for SAWs) and AlN (for FBARs).
Surface Acoustic Wave (SAW) Filters
The development of SAW devices dated back to 1965, when the first SAW devices were made. Earlier research work in SAW devices was largely to fulfill the needs of radar signal processing. In the 1980s and 1990s, the main development efforts were focused on low loss filters particularly for mobile phones. The basic principles of SAW devices can be understood by considering a basic SAW structure. FIG. 1A shows a schematic diagram of a prior art SAW filter (100) on a piezoelectric substrate (110), with an input inter digital transducer IDT1 (120) with a center-to-center distance between adjacent electrodes controlled to a “pitch” and connected to an electrical signal source (130) to excite acoustic waves (140) with a velocity v and at a frequency fo=v/(2×pitch), an output inter digital transducer IDT2 (150) with a center-to-center distance between adjacent electrodes again also controlled to the “pitch” to receive the acoustic waves (140) and to convert them into an output electrical signal (160). Electrical signals in the signal source (130) at frequencies other than fo cannot excite resonant acoustic waves in the input IDT1 (120) with sufficient level to reach the output IDT2 (150) and to generate an output in the output terminals. Once a SAW filter is fabricated, the central frequency fo of transmission and the bandwidth (BW) are fixed by the geometry of the filter and by materials used. The only electrical signals that are allowed to reach the output IDT from the input IDT are those with a frequency within the bandwidth of a center frequency fo.
The main properties of piezoelectric materials for filters are: propagation velocity of acoustic waves, electrode pitch and coupling coefficients, where the velocity of acoustic waves and the electrode pitch determine the resonant frequency and the coupling coefficients affect the bandwidth. Velocities values for several piezoelectric substrates are: LiNbO3˜4,000 m/s, ZnO ˜6,300 m/s, AlN ˜10,400 m/s and GaN ˜7,900 m/s. As an example, to obtain a filter on LiNbO3 with a central frequency fo of 2 GHz, the wavelength of the acoustic wave is λ=(4000 m/sec)/(2×109/sec)=2×104 cm. Therefore, the value of electrode pitch in FIG. 1 is then equal to (½)λ or 1 μm. Assuming that the width of electrodes and the space between adjacent electrodes are equal, the electrode width is then 0.5 μm.
Film Bulk Acoustic Wave Resonators (FBAR)
The basic element of the film bulk acoustic wave resonator (FBAR) is a thin film resonator which is very similar to the basic quartz crystal scaled down in size. FIG. 1B shows a schematic cross-sectional diagram of a FBAR (200) on a substrate (160) having a substrate thickness (160t), a piezoelectric film (180) of a thickness (180t) is sandwiched between two metal films (170, 190) having a thickness (170t, 190t respectively). An air cavity (165) having an air cavity depth (165t) is present to prevent the acoustic waves from getting into the substrate (160). The equivalent Butterworth/VanDyke circuit model consists of a fixed structure capacitance in parallel with a frequency dependant electro-mechanical resonant circuit. The key properties of the FBAR are set to store the maximum acoustic energy within the structure and to achieve a high electrical Q. The boundary conditions outside of the metal films must maintain a very high level of acoustic reflection with vacuum being the ideal interface. The materials chosen must optimize both electrical and mechanical properties.
Tunable Filters
For mobile communications, there are about 40 bands. More bands are expected for the next generation long term extension technology. For each communication band, there are two frequencies close to each other: one for transmitting and the other for receiving. Table 1 gives several selected bands for mobile communications used in different regions or countries. In each band, there is a transmit band or Tx Band at a transmit band central frequency foTR with a transmit bandwidth BWTR. There is also an associated receive band or Rx Band at a receive band central frequency foRE with a receive bandwidth BWRE. The separation between the transmit band and the receive band is given by: foRE−foTR.
TABLE 1Band frequencies and bandwidth for some of the Bands assigned to mobile handsetsand base stations.BandfoTR (MHz)BWTR (MHz)foRE (MHz)BWRE (MHz)foRE-foTR (MHz)Region11920-1980602110-217060190Asia, EMEA, Japan21850-1910601930-19906080N. America, Latin Am.31710-1785751805-18807595Asia, EMEA41710-1755452110-215545400N. America, Latin Am.5824-84925869-8942545N. America, Latin Am.72500-2570702620-269070120Asia, EMEA8880-91535925-9603545EMEA, Latin Am.12699-71617729-7461730N. America
Due to the large number of bands used in mobile handsets, a true world phone needs to cover all 40 bands, each with a transmit band and a receive band. Since each RF filter has only one fixed central frequency of resonant and a fixed bandwidth, therefore, such a true world phone will need to have 80 filters for the front end. Due to resource limitations, some designers design mobile phone handsets to cover 5 to 10 bands for selected regions or countries. Even with this reduced number of bands, the number of RF filters currently required is still large (10 to 20 units). Thus, it would be ideal to develop an RF filter which can cover as many bands or frequency ranges as possible so that the size and power consumption of RF front ends in a mobile handset and microwave systems can be reduced. In Table 1, values of (foRE−foTR)/foTR are listed. It is seen that majority has a value of 10% or less: mostly ˜5%. Therefore, tunable filters with a tuning range of 10% or more will be highly valuable for communications.
In order to fulfill the demands for RF filters covering as many bands or frequency ranges as possible, tunable SAW inter digital transducers and reflectors have been invented and disclosed in patent applications US2017-0085246 and US2017-0366165 by the inventors of the present application. These inventions provide tunable surface acoustic wave resonators utilizing semiconducting piezoelectric layers having embedded or elevated electrode doped regions. Both metallization ratio and loading mass are changed by varying a DC biasing voltage to effect a change in the resonant frequency. A plurality of the present tunable SAW devices may be connected into a tunable and selectable microwave filter for selecting and adjusting of the bandpass frequency or an tunable oscillator by varying the DC biasing voltages. In US patent applications US2017-0025596 and US2018-0069528, frequency tunable FBAR resonators and filters having at least a doped piezoelectric layer are disclosed. The central frequency of resonant is tuned by applying at least a DC biasing voltage.
Modern electronic systems such as: mobile phones, base stations and phase arrays often involve digital signals for computation, processing and representation of parameters such as frequencies. In an RF system involving a resonant frequency based on a voltage-controlled tunable filter, a voltage with a specific magnitude is required. Therefore, in order to use the voltage-controlled tunable RF filter in association with a modern electron system, there is a need to control the digital signals which represent the required resonant frequency and convert them into a DC voltage. This DC voltage is then applied to the voltage-controlled tunable microwave filter to vary the central frequency of the filter.